Thursday, June 5, 2014 Chapter 5: Condensation & Precipitation Chapter 5: Condensation and Precipitation Formation of Condensation Saturated Air Condensation Nuclei Results of Condensation Clouds Fog Dew Clouds Cloud Classification Cloud Descriptions Types of Fog Formed by Cooling Formed by Evaporation Precipitation Formation Collision/Coalescence Bergeron Process Types of Precipitation 1
Condensation The most important cloud formation process is Adiabatic Cooling As a parcel of air moves upward it experiences successively lower and lower pressure And because the pressure is lower, the air expands As air expands, it cools (lower internal density) Adiabatic Cooling: AIR RISES EXPANDS COOLS Adiabatic Cooling Adiabatic Cooling: AIR RISES EXPANDS COOLS [Adiabatic Warming]: AIR SINKS COMPRESSES WARMS The Rate is NEGATIVE as you rise (it gets cooler) The Rate is POSITIVE as you descend (it gets warmer) Dry Adiabatic Rate: (this happens both rising and descending) 10 C / 1000m or 5.5 F / 1000 Dry rate = NO CONDENSATION OCCURING Wet Adiabatic Rate (average): (this happens while RISING ONLY) 4 C / 1000m or 2.3 F / 1000 Wet Rate = Dry rate + RELEASE OF LATENT HEAT (from CONDENSATION) The wet rate VARIES with the latent heat content of the air 2
Condensation 1) The Air must be SATURATED Condensation Lifting Level Dew Point Temperature 100% Relative Humidity Saturation 2) Condensation Nuclei are needed Cloud Condensation Nuclei It takes 1,000+ cloud droplets to make one raindrop There needs to be a SURFACE onto which water vapor (gas) can condense. On the ground, dew forms when condensation forms on blades of grass In the atmosphere, tiny particles known as Condensation Nuclei serve as these surfaces Condensation Nuclei include: Microscopic dust Smoke Salt Bacteria (recently reported in Science) 3
Cloud Classification: Form Cirrus Clouds High, white and thin. Separate and detached Form delicate streaks or wispy fibers Seem to have a feathery appearance Cumulous Clouds Globular individual cloud masses Often having a flat base and rise in towers Seem to look like mashed potatoes Stratus Clouds Look like sheets or layers Most often cover the entire (or most of) the sky Can be minor breaks, appear to be one continuous formation Cirrus 4
Cumulus Stratus 5
Cloud Classification: Height High Clouds Bases at or above 6,000m (20,000 ft) Cirrus Cirrostratus Cirrocumulus Low temperatures and little water vapor Clouds appear thin and white Composed mostly of Ice Crystals Middle Clouds Bases between 2,000 6,000m (6,500 20,000 ft) All have ALTO as a prefix (middle) Altocumulus Altostratus Similar to high clouds, but mostly composed of liquid water (not ice) Low Clouds Bases below 2,000m (6,500 ft) Stratus Stratocumulus Nimbostratus More turbulence near surface produces a more chaotic mix Clouds of Vertical Development Bases in low range, but extend upward into middle or high ranges Cumulus Cumulonimbus Associated with instability, these clouds continue to rise and often are accompanied by precipitation Nimbus = Rain Cloud Descriptions High Clouds (over 6,000m) Middle Clouds (2,000-4,000m) Low Clouds (under 2,000m) 6
High: Cirrus High: Cirrostratus 7
High: Cirronimbus Middle: Altocumulus 8
Middle: Altostratus Low: Stratus 9
Low: Stratocumulus Low: Nimbostratus 10
Cumulus Cumulonimbus 11
Cumulonimbus 12
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Types of Fog Types of Fog Formed by cooling Formed by evaporation Dew and Frost Precipitation Formation Collision / Coalescence The Bergeron Process Forms of Precipitation Rain Snow Hail Sleet Glaze Rime Weather Modification 15
Fog Fog: A cloud with its base at or very near the ground Fogs formed by Cooling: Radiation Fog: Fog resulting from the radiation cooling of the ground and the adjacent air. Advection Fog: Fog resulting when warm moist air is blown over a cold surface Upslope Fog: Fog resulting from orographic uplift and adiabatic cooling Fogs formed by Evaporation: Steam Fog: Fog resulting when cold air moves over a warm lake Frontal Fog: Fog resulting from rain evaporating in cold air, and condensing as fog Fog 16
Fog Fog 17
Dew and Frost Dew: Condensation on surfaces that are below the dew point temperature of the surrounding air Frost: Deposition (gas to solid) of water vapor onto surfaces that are below freezing. Frost is NOT frozen Dew! Precipitation Formation Condensation does NOT produce rain! Condensation produces CLOUDS! Precipitation forms within clouds in two ways In Warm Clouds: Collison / Coalescence In Cold Clouds: The Bergeron Process 18
Collision / Coalescence ~ warm cloud process~ Collision/Coalescence: Cloud droplets are in motion within the cloud These droplets randomly collide with other droplets They coalesce (join together) and form a larger droplet This process continues until a rain drop forms (big enough to fall) The Bergeron Process ~ cold cloud process~ The Bergeron Process: The cloud (at least the tops) are below freezing Cloud droplets do not freeze at 0 C (it is closer to -40 C!) This property of water is called supercooled water Supercooled water will freeze on contact with an object (freezing rain/glaze) 19
The Bergeron Process ~ cold cloud process~ The Bergeron Process: Freezing Nuclei are needed to form precipitation in the cloud (Note: These are NOT the same as Condensation Nuclei) The supercooled cloud droplets will evaporate into the air and then deposit (gas solid) onto the freezing nuclei This forms snow crystals Forms of Precipitation Rain: Drops of water that fall from a cloud Snow: Ice crystals (snow flakes) or clusters of crystals that fall from a cloud Sleet: Frozen raindrops. Occurs when liquid rain falls through a cold lower layer and the drops freeze Glaze: Occurs when liquid rain falls onto objects that are below freezing (or the air near the ground is below freezing), causing a coating of ice to form on the objects Hail: Frozen ball of ice formed in the updrafts of a powerful cloud. Multiple trips up and down through the cloud produce semi-concentric layers of ice. Largest US hailstone = 7 + diameter = volleyball-sized Rime: Deposition (gas solid) forms from supercooled fog or cloud droplets onto below freezing surfaces 20
Forms of Precipitation Rain: Snow: Sleet: Glaze: Hail: Rime: Forms of Precipitation Rain: Drops of water that fall from a cloud Snow: Ice crystals (snow flakes) or clusters of crystals that fall from a cloud Sleet: Frozen raindrops. Occurs when liquid rain falls through a cold lower layer and the drops freeze Glaze: Occurs when liquid rain falls onto objects that are below freezing (or the air near the ground is below freezing), causing a coating of ice to form on the objects Hail: Frozen ball of ice formed in the updrafts of a powerful cloud. Multiple trips up and down through the cloud produce semi-concentric layers of ice. Largest US hailstone = 7 + diameter = volleyball-sized Rime: Deposition (gas solid) forms from supercooled fog or cloud droplets onto below freezing surfaces 21
Forms of Precipitation Rain: Snow: Sleet: Glaze: Hail: Rime: Forms of Precipitation Rain: Drops of water that fall from a cloud Snow: Ice crystals (snow flakes) or clusters of crystals that fall from a cloud Sleet: Frozen raindrops. Occurs when liquid rain falls through a cold lower layer and the drops freeze Glaze: Occurs when liquid rain falls onto objects that are below freezing (or the air near the ground is below freezing), causing a coating of ice to form on the objects Hail: Frozen ball of ice formed in the updrafts of a powerful cloud. Multiple trips up and down through the cloud produce semi-concentric layers of ice. Largest US hailstone = 7 + diameter = volleyball-sized Rime: Deposition (gas solid) forms from supercooled fog or cloud droplets onto below freezing surfaces 22
Forms of Precipitation Rain: Snow: Sleet: Glaze: Hail: Rime: Forms of Precipitation Rain: Drops of water that fall from a cloud Snow: Ice crystals (snow flakes) or clusters of crystals that fall from a cloud Sleet: Frozen raindrops. Occurs when liquid rain falls through a cold lower layer and the drops freeze Glaze: Occurs when liquid rain falls onto objects that are below freezing (or the air near the ground is below freezing), causing a coating of ice to form on the objects Hail: Frozen ball of ice formed in the updrafts of a powerful cloud. Multiple trips up and down through the cloud produce semi-concentric layers of ice. Largest US hailstone = 7 + diameter = volleyball-sized Rime: Deposition (gas solid) forms from supercooled fog or cloud droplets onto below freezing surfaces 23
Forms of Precipitation Rain: Snow: Sleet: Glaze: Hail: Rime: Forms of Precipitation Rain: Snow: Sleet: Glaze: Hail: Rime: 24
Forms of Precipitation Rain: Drops of water that fall from a cloud Snow: Ice crystals (snow flakes) or clusters of crystals that fall from a cloud Sleet: Frozen raindrops. Occurs when liquid rain falls through a cold lower layer and the drops freeze Glaze: Occurs when liquid rain falls onto objects that are below freezing (or the air near the ground is below freezing), causing a coating of ice to form on the objects Hail: Frozen ball of ice formed in the updrafts of a powerful cloud. Multiple trips up and down through the cloud produce semi-concentric layers of ice. Largest US hailstone = 7 + = volleyball-sized Rime: Deposition (gas solid) forms from supercooled fog or cloud droplets onto below freezing surfaces Hail Rain: Snow: Sleet: Glaze: Hail: Rime: 25
Forms of Precipitation Rain: Snow: Sleet: Glaze: Hail: Rime: Largest US Hailstone In South Dakota Weight: 1.9 pounds Diameter: 8.1 inches Circumference: 18.8 inches 26
Forms of Precipitation Rain: Drops of water that fall from a cloud Snow: Ice crystals (snow flakes) or clusters of crystals that fall from a cloud Sleet: Frozen raindrops. Occurs when liquid rain falls through a cold lower layer and the drops freeze Glaze: Occurs when liquid rain falls onto objects that are below freezing (or the air near the ground is below freezing), causing a coating of ice to form on the objects Hail: Frozen ball of ice formed in the updrafts of a powerful cloud. Multiple trips up and down through the cloud produce semi-concentric layers of ice. Largest US hailstone = 7 + diameter = volleyball-sized Rime: Deposition (gas solid) forms from supercooled fog or cloud droplets onto below freezing surfaces Forms of Precipitation Rain: Snow: Sleet: Glaze: Hail: Rime: 27
Weather Modification Intentional methods of modifying the Weather Cloud Seeding: Intentional injection of condensation nuclei (or freezing nuclei) into a cloud to induce precipitation (usually for a beneficial outcome, i.e. ease drought conditions) Fog and Cloud Dispersal: Attempts to induce precipitation in order to remove the droplets from the air (often as an aid to visibility) Hail Suppression: Attempts to limit or stop the formation of hail within large cloud banks Frost Prevention: Involve either heating the air near the ground, or insulating the objects needing protection (often fruit crops) Spraying water can prevent fruit freezing. ONLY PROVEN METHOD! 28